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Author Topic: Smoothing full wave rectified exciter current Clive Carmock Film Handler Posts: 24 From: Morden, Surrey, UK Registered: Dec 2004 posted 04-04-2005 08:09 PM                     My TK 35 was modifed long before I got it to run natively on 230V UK mains from the original 110V. There is a 230V to 6V step down transformer fitted inside for the exciter supply. The output of this is connected to a bridge rectifier which then connects directly to the exciter lamp. Ideally the exciter supply would be true DC, but it is getting a full wave rectified supply. Given the latency inherent in the lamp this doesn't produce any mains hum on the audio. However I want to change the exciter for a replacement RED LED exciter, which is made for this old machine. Given that is a solid state device I will need to supply it with true smoothed DC. Can I simply connect a capacitor across the exciter supply after the bridge rectifier? If so I wonder if anyone could advise me of a suitable size capacitor. Regards Clive  |  IP: Logged Randy Stankey Film God Posts: 6539 From: Erie, Pennsylvania Registered: Jun 99 posted 04-04-2005 11:20 PM                     That supply, being completely unregulated, may be OK for an incandescent lamp but it's really not suitable for an LED. At 6 volts and with no current regulation to speak of, an LED is likely to just go "POP!". If what you want to do is simply substitute an LED for the incandescnet you could go to an electronics supply store and buy a "Wall Wart" power supply of the correct voltate/current characteristics and use that. It's a whole lot easier to let somebody ELSE design your power supply for you. Nobody but hard-core Techno Geeks design their own PS any more. The problem with that idea is that just substituting the LED for the old lamp is likely to give crappy results at best. Regular solar cells aren't sensitive enough to red light. You'll have to put a nasty-ass high gain preamp on the output of the cell to get even a partially decent signal out of it. In so doing, you not only amplify the signal but any noise along with it. JaxLites are/were an attempt to do almost exactly what you are thinking about doing. JaxLites SUCK! The best thing to do is try to get a red LED conversion kit for your projector. It will provide you with the proper LED. (It's not a standard LED.) You will also get a CCD sound pickup to replace your solar cell which will include all the preamps and internal electronics to make it work. Add in your own regulated power supply or buy one from the manufacturer and you'll be all set. However, if the point of this excercise is just to experiment and see how well your projector works with different light source(s) in the sound head I suggest the ready made supply from the electronics store. Make sure the supply's voltage is within the limist of the LED's specs. (Usually 2-5 volts.) Put a resistor or a potentiometer in-line with the LED. You could cannibalize a power supply from some long abandoned electronic device from around your house, like an old cellular phone you don't use anymore. Just check the output characteristics and match them to the LED. If you just want to make an adventure into the afformentioned hard-core geekdom, don't simply filter the output of the FWBR. Use a regulator. Not only will you get virtually ripple free output that doesn't vary when load is applied/removed, if you choose the right kind of regulator, you will also have overcurrent/short circuit protection too. Take a look HERE for an example of a regulated power supply that you can build yourself.  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 04-04-2005 11:54 PM                       I want to correct one thing Randy states. All Reverse scan readers including mine (BACP)use a solar cell designed to work in a reverse scan application, not a CCD. As to filtering ripple on a power supply intended for use with an LED used to light an analog soundtrack; you need to get the peak to peak ripple below .5 mVolts before it is inaudible. Maximum current of the Dolby LED array is 300 mA with a substantial heatsink for reasonable LED life.  |  IP: Logged Randy Stankey Film God Posts: 6539 From: Erie, Pennsylvania Registered: Jun 99 posted 04-05-2005 09:00 AM                     Hmmm... Solar cell, eh? I thought they had gone to CCD. A while back, there were a couple of Comp. Eng. readers that went bad on me so I took one apart to see what made it tick. Inside, there was what looked like a multi-pin (16 pin, I think.) DIP IC chip with a window in it. I simply assumed it was a CCD. Could it have been a custom designed solar cell that was simply mounted on an IC for convenience? I stand corrected, then. No matter what the modus operandi when you buy the conversion kit you get almost everything you need.  |  IP: Logged Clive Carmock Film Handler Posts: 24 From: Morden, Surrey, UK Registered: Dec 2004 posted 04-05-2005 03:42 PM                     Thanks guys, as always this place is a source of excellent advice. Randy I agree with you and wish that a full conversion kit was available for my machine. I really do need to upgrade to a well known make of full sized machine, which is my ultimate goal. However the TK 35 is a portable machine made in the late 50's in East Germany. I don't know of a proper full conversion kit for it. So I was kind of planning to experiment a bit. Witner Kinoteknik in Germany have their own equivalent of Jaxlights, adn do have one for this machine which I was planning to try. I am concerned about the solar cell output given the age of this machine, I suspect it is a solar cell as opposed to photo diode (or is it the other way round!). I am attaching a picture of the soundhead on this machine which might make things a little clearer. Clive  |  IP: Logged Randy Stankey Film God Posts: 6539 From: Erie, Pennsylvania Registered: Jun 99 posted 04-05-2005 04:07 PM                     Is this machine something you want to preserve in as original condition as possible, or do you want to tinker with it? If you want to preserve it, get the JaxLite. (Or equivalent.) If you want to tinker, you have little to lose by experimenting. After the regulated supply, the main thing you'll need is a preamplifier. If you're into tinkering, download a schematic off the WWW and see what you can cook up. When I was in Tech School, we had to make a power supply and amplifier completely out of vacuum tubes as our final grade. But, alas, that was nearly 25 years ago! Nowadays, I just go to Radio Shack and buy what I need.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 04-05-2005 07:40 PM                     Randy, Dolby Digital readers DO use an CCD array. However, analog readers still use the good ol solar cell. As to lighting an LED from an Exciter supply...it is certainly doable....the question is, is it worth it. I wouldn't necessary just grab a wall-wart either...they normally supply a proper supply. Most Wall-Warts are not regulated anyway. In order to drive an LED...any LED...you need to know its parameters. That is, its V-gamma and current. The V-gamma is its forward conduction voltage. Using this and its current demands, one can choose appropriate components to turn a higher power supply into a lower power supply for the LED. You are going to want good regulation and filtration though. And you will also want some current control to vary its intensity to always having it operate in its optimum parameters. For me, I've always used what the manufacturer's have supplied though there is one that I probably won't use again since the supply design was a bit too off-the-shelf to the point of oscillations and poor current range.  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 04-05-2005 08:28 PM                       Steve, Whatever are you referencing in yur last paragraph?  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 04-06-2005 07:44 AM                     Only that there is one supply that I've come across that is a bit too on the edge and has very poor current range.  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 04-06-2005 11:12 AM                       Steve, Are you going to name names?  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 04-06-2005 06:00 PM                     Nope....I probably shouldn't have brought it up...in retrospect.  |  IP: Logged

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